Simplified AC-DC switching converter with output isolation

Abstract

An AC-DC switching converter converts AC mains power into regulated DC power into a load. It is desired to electrically isolate the load from the mains, and this requires isolation in both the forward power transfer path and the reverse feedback path. The forward path is isolated by use of a transformer, and the feedback signal is transferred through an optocoupler. In a typical pulsewidth modulation control loop design, it is difficult to achieve good loop dynamics when using low cost optocouplers, because of the wide variation of their transfer function gain. However, in the disclosed invention, the loop is operated as a nonlinear limit cycle ("bang--bang") controller. In this mode, the optocoupler gain variation has an insignificant effect on loop operation. The converter switching signal is provided by an inexpensive 555 timer oscillator, and the optocoupler is connected to the reset pin of this timer to turn it ON and OFF in short bursts, as is characteristic of this type of controller. Since a converter must operate over a mains voltage range of about 3:1, the disclosure also includes a technique for protecting the transformer from possible saturation by narrowing the oscillator pulses as an inverse function of the mains voltage.

We claim: 
     
       1. In an AC-DC converter for converting an AC mains voltage input into a DC output voltage across a load, the converter being of the pulse-switching, limit-cycle type, and the load being electrically isolated from the AC mains, a method of regulating the DC output voltage, comprising the steps of: sensing the DC output voltage with the input of a comparator having input-output isolation;   coupling the output of the comparator to an ON/OFF control input of an oscillator which provides a switching signal for the AC-DC converter; and   adjusting the comparator to turn OFF the oscillator whenever the DC output voltage exceeds a threshold region around a predetermined voltage value and, conversely, to turn ON the oscillator whenever the DC output voltage falls below the threshold region.   
     
     
       2. In an AC-DC converter as recited in claim 1, the comparator further comprising an opto-coupler circuit. 
     
     
       3. In an AC-DC converter as recited in claim 1, the further steps of: sensing a DC voltage proportional to the AC mains voltage;   adjusting the pulsewidth of the oscillator according to an inverse function of the magnitude of the DC voltage.   
     
     
       4. In an AC-DC converter as recited in claim 3, the sensing step further comprising generating a direct current proportional to the AC mains voltage, and the adjusting step further comprising controlling, with the direct current, the charge on a timing capacitor of the oscillator, thereby reducing the oscillator pulsewidth for increased direct current. 
     
     
       5. in an AC-DC converter as recited in claim 4, the adjusting step further comprising supplying the direct current to the timing capacitor. 
     
     
       6. In an AC-DC converter as recited in claim 4, the adjusting step further comprising supplying the direct current to the input of a current mirror whose output current is supplied to the timing capacitor. 
     
     
       7. In an AC-DC converter for converting an AC mains voltage input into a DC output voltage across a load, the converter being of the pulse-switching, limit-cycle type, having an oscillator for switching rectified AC mains voltage through a transformer coupled to the load, a method of limiting the transformer magnetizing current at high mains voltages, comprising the steps of: sensing the rectified AC mains voltage; and   adjusting the pulsewidth of the oscillator according to an inverse function of the magnitude of the rectified AC mains voltage.   
     
     
       8. In an AC-DC converter as recited in claim 7, the oscillator further having an input responsive to direct current control signals for adjusting its pulsewidth, such that increasing a direct current control signal to this input reduces the pulsewidth. 
     
     
       9. In an AC-DC converter as recited in claim 8, the sensing step further comprising generating a direct current proportional to the rectified AC mains voltage, and the adjusting step further comprising supplying the direct current to the input of the oscillator. 
     
     
       10. In an AC-DC converter as recited in claim 8, the sensing step further comprising generating a direct current proportional to the rectified AC mains voltage, and the adjusting step further comprising supplying the direct current to an input of a current mirror whose output is supplied to the input of the oscillator. 
     
     
       11. An AC-DC converter of the pulse-switching, limit-cycle type for converting an AC mains input voltage into a DC output voltage across a load, the load being electrically isolated from the AC mains, the converter comprising: a bridge circuit for converting the AC mains voltage to a high voltage DC, having an input connected to the mains, an output, and a ground return;   a transformer having a first side of a primary winding connected to the bridge output, a second side of the primary winding, and a secondary winding;   a SPST switch connected between the second side of the primary winding and the ground return, and having a control input;   an oscillator circuit having a pulse output coupled to the control input of the switch, and an ON/OFF control input for governing the oscillation of the oscillator circuit;   a rectifier and filter circuit coupled to the transformer secondary for converting pulsed secondary voltage into the DC output voltage; and   a comparator circuit providing electrical isolation between an input coupled to the DC output voltage, and an output coupled to the oscillator ON/OFF control input; whereby the comparator circuit, sensing the proximity of the DC output voltage to a predetermined voltage value, switches the oscillator ON and OFF, causing the DC output voltage to vary continually in a small range around the predetermined voltage value.     
     
     
       12. An AC-DC converter, as recited in claim 11, wherein the comparator circuit comprises an opto-coupler. 
     
     
       13. An AC-DC converter, as recited in claim 11, further comprising: the oscillator circuit having, in addition to the ON/OFF control input, a second control input for controlling the oscillator pulsewidth; and   a sensing circuit, having an input coupled to the high voltage DC and an output coupled to the second control input of the oscillator for controlling its pulsewidth; whereby the oscillator pulsewidth decreases as the mains voltage increases.     
     
     
       14. An AC-DC converter, as recited in claim 13, wherein the sensing circuit comprises a current mirror. 
     
     
       15. A DC-DC converter of the pulse-switching, limit-cycle type for converting energy input from a DC source, having high level and low level terminals, into a DC output voltage across a load which is electrically isolated from the DC source, the converter comprising: a transformer having a first side of a primary winding connected to the high level terminal, a second side of the primary winding, and a secondary winding;   a SPST switch connected between the second side of the primary winding and the low level terminal, and having a control input;   an oscillator circuit having a pulse output coupled to the control input of the switch, and an ON/OFF control input for governing the oscillation of the oscillator circuit;   a rectifier and filter circuit coupled to the transformer secondary for converting pulsed secondary voltage into the DC output voltage; and   a comparator circuit providing electrical isolation between an input coupled to the DC output voltage, and an output coupled to the oscillator ON/OFF control input; whereby the comparator circuit, sensing the proximity of the DC output voltage to a predetermined voltage value, switches the oscillator ON and OFF, causing the DC output voltage to vary continually in a small range around the predetermined voltage value.   
     
     
       16. A DC-DC converter, as recited in claim 15, further comprising: the oscillator circuit being of a type having a controllable pulsewidth; and   a sensing circuit, having an input coupled to the DC source and an output coupled to the oscillator for controlling its pulsewidth, whereby the pulsewidth decreases as the DC source voltage increases.   
     
     
       17. A DC-DC converter, as recited in claim 15, wherein the comparator circuit comprises an opto-coupler.